Isolated mammalian muscle spindles are used to study the mechanisms of sensory transduction and motor regulation of this sense organ. This supplemental application is concerned particularly with the use of electron microscopy for correlative studies of structure and function. These involve the fine structure of the sensory membrane, the access of the cleft between sensory terminal and intrafusal fiber to extracellular space and the correlation of Nomarski images with electron micrographs of the same structures.